Werewolves, scientifically known as lycanthhropes, (thropes) are roaming the city killing innocent people who just happen to be in the wrong place at the wrong time. But these Links' superhuman senses and strength, and their ability to blend in with the human population while not in wolf form, make the detectives' work quite intense...

A Link is formed out of fat from liposuction patients from a plastic surgery clinic. The link kills the plastic surgeon when he gets in the way of the link's food supply and then disappears into the Chicago water system. The first detective on the scene is Rich Talridge, who was O'Malley's first partner before Nick was picked for SU2. Rich doesn't like that the case has been handed over to SU2 and tries to solve it himself which gets him in trouble. Kate and Nick and the rest of SU2 are left trying to track and get rid of the "Fat Link" before it kills again.

Kate and Nick are forced to release the original Chameleon that was imprisoned by the SU2 team so he will help them catch a new Chameleon who is now terrorizing Chicago. Nick is in a bad mood because the original Chameleon that has to help them killed his former lover.

A Link so monumentally evil that his name has been lost to history reappears on Halloween, the one night of the year when Links can roam with impunity. The confrontation between the Link and Nick is made all the more personal - and lethal - as it is the villain responsible for the death of Nick's former partner. The mysterious Link is bringing the corpses of serial killers back to life so they can carry out his plan to assume leadership of all Links.

When Cupid's 'love potion' causes a hangin' judge to start setting high-end criminals free, it's up to Kate and Carl to get an on-the-lam Cupid back to Chicago to reverse his potion's' effect on both the judge and a transformed Nick, whose newfound propensity for seeing the good in everyone begins to wear thin on his colleagues.